As a teacher , I know that movement is a great way to help children with focus. When your child gets home allow them some time to unwind. Have a snack & take a bike ride. Remember he or she as been sitting all day. Make a special place for your child to work. Add some fun pencils or manipulatives for he or she to work with. Add a timer and break the work load up into smaller chunks. Even highlighting the problems one at a time will help your child not to feel so overwhelmed. Check out my blog post about Laser Fingers. Adding those in as a reward for working can turn any homework into FUN work!
Another easy fix is to allow them to stand or sit on that exercise ball that is hiding in your house. Give your child frequent movement brakes like table top push ups or even jumping jacks. With all of that being said, don't do the work for them. It is so easy to help them along with the answers to get the assignment done. Teachers are not always looking for completion just practice of a specific skill. Ask your child's teacher to shorten the assignment a bit so that your child can have a feeling of success.
If all else fails...write a note on the assignment and let the teacher know the issues that you had. Communication is the key to success with homework.